1) Field of the Invention
The present invention relates to a lighting device, an image reading apparatus, and an image forming apparatus.
2) Description of the Related Art
An image reading apparatus, such as a scanner, has a light source. This light source is located above a contact glass and emits rays toward a surface of a document to be scanned. The light reflected from the surface of the document is focuses on a photoelectric conversion element (CCD) via a lens.
As a light source for such an image reading apparatus, a bar light source like a fluorescent tube or a xenon lamp or a point light source such as an LED is used.
When the bar light source is used, flare light tends to be generated because a width of emission of rays from the bar light source is wider in a sub-scanning direction when reading the document. Rays reflected on a surface of a document in directions other than a direction along a reading optical axis is reflected on the bar light source and members around the bar light source. The flare light is the reflected light traveling toward the surface of the document again. Such flare light is generated more often when the surface of the document is more whitish. Therefore, an amount of generation of flare light is different when the surface of the document is whitish and blackish, and decline in reading performance due to an influence of the flare light is caused.
On the other hand, when the point light source, such as an LED, is used, for example, those disclosed in Japanese Patent Application Laid-Open Publication Nos. H08-116401, H10-126576, and H10-336392 are known. Since the point light source such as an LED has a smaller quantity of light compared with the bar light source, generation of the flare light is small, and the decline in reading performance due to the influence of the flare light is controlled.
When the point light source is used as a light source, plural point light sources are arranged linearly in a main scanning direction (reading line direction). Thus, when rays emitted from the respective point light sources are directly irradiated on a reading surface like a surface of a document, an illuminance ripple (unevenness of an illuminance distribution) occurs in the main scanning direction. As a result, when an image is formed according to read image data, unevenness in image concentration due to the illuminance ripple occurs.
Therefore, various techniques for controlling the occurrence of an illuminance ripple in the image reading apparatus using the point light source have been proposed. For example, according to Japanese Patent Application Laid-Open Publication No. H10-322521, illuminance equalizing means including a diffraction grating is provided between plural point light sources and a surface of a document that is an object of reading.
However, since the point light sources have a small quantity of light, insufficiency of a quantity of light tends to occur unless a direction of irradiation of light is considered sufficiently.
In addition, since the point light sources have a small quantity of light, the point light sources tend to be arranged close to a surface of a document. Therefore, when the surface of the document lifts from a contact glass, a rate of decline in a quantity of light irradiated on the surface of the document is large even if a dimension of the lift is small, and reading performance tends to fall.
When a cut and pasted document is read, if the document is lighted only from one direction of a reading optical axis, a shadow occurs in a cut and pasted step part.
Japanese Patent Application Laid-Open No. H10-322521 discloses a structure for diffracting rays emitted from the point light sources with the diffraction grating. Thus, although a difference between a maximum illuminance and a minimum illuminance of the illuminance ripple is reduced, the image reading apparatus comes into a state in which the number of ripples has increased, and the illuminance ripple still remains.